Infrared spectroscopy (and radiometry) has been accepted as an analytical technique for the analysis of solids, liquids and gasses. In general, infrared-absorption spectroscopic analysis of a sample involves a source of infrared (IR) analytical radiation, a detector or analyzer, and optical transmitting means for directing the analytical radiation from the source to the sample (which reflects and absorbs portions of the radiation), and for directing reflected or unabsorbed portions of the radiation from the sample to the detector. Alternatively, infrared-emission spectroscopic analysis uses the sample itself as a source of analytical radiation. By analyzing the returning or emitted beam of radiation in these techniques, and in particular the absorption pattern of the beam, certain absorption (or emission) characteristics of the sample can be determined.
In order to perform infrared spectroscopic analysis, it is usually necessary to use ITMs to contain or bound the sample (when performing transmission and/or emission experiments), or to contact the sample (when performing internal-reflection spectroscopic experiments). Since an ITM is typically in physical contact with the sample, the optical properties of the ITM affect the radiation received by the detector. When an ITM does not transmit energy of specific wavelength regions, the ITM is said to be opaque in that region. When used to contain, bound, or contact a sample for spectroscopic analysis, spectroscopic information about the sample cannot be obtained in the opaque spectral region. In order to obtain spectroscopic information when using an ITM, the ITM must transmit sufficient radiant energy through the specific wavelength regions to allow radiant energy from the source to reach the sample and transmit that radiant energy (after encoding with the sample's absorption information) so that it reaches the detector.
Since many ITMs are in physical contact with the sample, an ITM must be chemically inert to the sample under the pressure, temperature, and flow conditions imposed on both the sample and the ITM during analysis. For example, the ITM must resist abrasion, scratching, corrosion and stress from the sample during analysis. In addition, the surface properties and qualities of an ITM are important. If a sample interacts with an ITM, the sample may form unacceptable precipitates or thin films on the ITM's surface.
Different ITMs have different chemical and mechanical properties. Typically, it is necessary to match the chemical and mechanical properties of the ITM to those of the sample or class of samples being analyzed. However, it is believed that there is no one material that exhibits all of the desirable qualities for an ITM. For example, zinc selenide (ZnSe) is a widely-used ITM for internal-reflection spectroscopy (IRS). It has relatively broad optical-transmission properties [20,000-500 cm.sup.-1 (0.5-20 .mu.m)], has a desirable refractive index for IRS [2.40 at 1,100 cm.sup.-1 ], and is modest in cost when used in analytical laboratories with a large number of routinely-used solvents and products. In addition, it has an acceptable lifespan when treated with care using standard laboratory practices.
Unfortunately, ZnSe is soft when compared to materials such as laboratory glassware, is easily abraded when placed in contact with harder materials, and is easily degraded when subjected to most solvents under elevated temperatures and pressures or to strong acids or bases.
Another widely-used ITM is KRS-5 (TlBr-TlI). Again, this ITM exhibits highly-desirable optical properties, but is very soft and easily damaged or degraded with normal use.
On the other hand, diamond is a material with highly-desirable mechanical properties and chemical resistivity. However, diamond is relatively expensive and is available in only limited sizes and shapes (except at very high costs).
Other materials used for ITMs include Amtir (glass), arsenic-modified selenium glass (SeAs), cadmium sulfide (CdS), cadmium telluride (CdTe), cesium iodide (CsI), diamond (C), germanium (Ge), indium antimonide (InSb), silicon (Si), sapphire (Al.sub.2 O.sub.3), silver bromide (AgBr), silver chloride (AgCl), sulfur (S), sulfur-selenium glasses (S.sub.x Se.sub.y), thallium bromide (TlBr), thallium chloride (TlCl), zinc sulfide (ZnS), zirconia (ZrO.sub.2, cubic), sodium chloride (NaCl), potassium bromide (KBr), and potassium chloride (KCl), among others. However, there is no known ITM with all of the qualities required for a wide range of uses at acceptable costs.
Diamond-like coatings (DLCs) have been deposited on materials such as ZnSe or ZnS to provide an ITM window. However, it is believed that the current state of the art (as known to the inventors) is inadequate to provide commercially-acceptable and economical composite ITMs, and in particular, it is believed that DLCs cannot withstand the chemical and mechanical attacks normally required of ITMs, and are prone to crazing or peeling in some situations. These problems appear to be largely due to the differences in the mechanical properties of the coefficient of thermal expansion of the two or more materials bonded together in the composite and to the imperfections of the coatings of the materials.
Over the years, attempts have been made to develop new composite ITMs by using chemical-vapor deposition or other techniques to bond two or more materials together. While it is believed that a certain amount of success has been achieved in the development of some composite ITMs, there is no widely-acceptable composite ITM available to date.
The deposition of a sample on the surface of windows or internal reflection elements (IRE) is a related problem where composite ITMs could be used. Removal of deposits often requires aggressive mechanical or chemical cleaning that can damage the ITM. Using a removable and disposable plate of the same material as the IRE was a solution to this problem proposed by Gerhard J. Muller, "Spectroscopy with the Evanescent Wave in the Visible Region of the Spectrum", Multichannel Image Detectors, American Chemical Society, 1979 (hereinafter "Muller"). This prior art teaches the use of liquids to optically couple two solid optical elements of the same material to form a temporary composite internal reflection spectral analysis unit for use with visible light. Using liquids to achieve optical contact is a common practice for visible radiation, since transparent liquids are readily available in this spectral range. However, in the infrared spectral region transparent liquids are not available. Muller does not teach or suggest any method or means for optically coupling ITMs in the infrared spectral region. To the inventors' knowledge, bromine and carbon diselenide are the only liquids that would be transparent in the mid-infrared spectral region. Sulfur-selenium mixtures melt below 150.degree. C. and form glasses on cooling. These melts are transparent in the mid-infrared, but the glasses crystallize and are not reliable as optical coupling materials. Diamond is an ITM that can withstand aggressive cleaning, but large, thick diamond windows or IREs are very expensive and therefore impractical for most applications.
Hence, until now, there have been certain drawbacks in providing an ITM which simultaneously has a broad (or selected) optical transmission range; chemical resistivity to the sample; appropriate mechanical strength; and acceptably low cost, for use in spectroscopic (or radiometric) analysis.